Contact and non-contact forces — AQA GCSE Combined Science
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Contact and non-contact forces explained
A force is a push or a pull acting on an object because it interacts with another object; forces arise from interactions and are measured in newtons (N).
Read the full explanation
Forces between objects are classified as contact or non-contact. Contact forces require the objects to be touching, for example friction, air resistance, tension and normal contact force. Non-contact forces act at a distance without touching, for example gravitational force, electrostatic force and magnetic force. Every force has a direction and magnitude, so it is a vector. When identifying a force, name the two interacting objects and state whether they touch, then classify the force accordingly.
contact forces – the objects are physically touching
A contact force is any force that acts between two objects only because their surfaces or bodies are physically touching. The interaction is transmitted at the point of contact, so if the objects are separated, the force disappears. Friction, air resistance, water resistance, tension in a rope, normal contact force from a floor and the push of a hand on a box are all contact forces. To identify one, trace the force to the touching surfaces: for example, a book resting on a table pushes down on the table and the table pushes up on the book, and both forces require the surfaces to meet. When drawing free-body diagrams, contact forces are drawn at the contact point, whereas non-contact forces act at a distance.
non-contact forces – the objects are physically separated.
A non-contact force acts between objects that are physically separated; the objects do not need to touch for the force to be exerted. Gravity, electrostatic force and magnetic force are the main non-contact forces met at GCSE. For example, a magnet can pull a steel paper clip across a gap, and the Earth pulls a falling apple without touching it. The strength of a non-contact force usually decreases as the separation increases, although you do not need to calculate that relationship here. To identify a non-contact force, check whether the effect still happens when the objects are apart. In free-body diagrams, non-contact forces are drawn acting on the object even when no other object is in contact with it.
Examples of contact forces include friction, air resistance, tension and normal contact force.
A contact force acts only where two objects or surfaces touch, so removing the contact removes the force. Friction opposes relative sliding between surfaces, for example a book dragged across a desk. Air resistance is a form of friction acting on an object moving through air, opposing its motion. Tension is the pulling force transmitted along a stretched string, rope or cable, acting towards the object at each end. The normal contact force acts perpendicular to a surface, supporting an object resting on it; on a horizontal table it balances the object's weight. To identify a contact force, ask whether the objects must touch for the force to exist. If they must, it is a contact force.
Examples of non-contact forces are gravitational force, electrostatic force and magnetic force.
A non-contact force acts between objects that are not touching, because it acts at a distance through a field. Gravitational force attracts any two masses; the Earth pulls a falling ball downwards even before it lands. Electrostatic force acts between electric charges: like charges repel and opposite charges attract, as when a charged balloon attracts small paper pieces. Magnetic force acts between magnetic poles or between a magnet and a magnetic material; like poles repel and opposite poles attract. To classify a force, ask whether the objects must touch. If they can interact while separated, the force is non-contact. Field strength decreases with distance, so the force weakens as separation increases.
Force is a vector quantity.
A vector quantity carries both a size and a direction, and force is one of these because its effect depends on where it pushes or pulls as well as how hard. A 20 N push to the right and a 20 N push to the left have equal magnitude but opposite direction, so they do not cancel to zero unless you account for direction. When several forces act, you add them as vectors: forces along the same line combine by direction, and forces at an angle need a scale drawing or calculation. This is why a free-body diagram shows each force as an arrow whose length represents magnitude and whose head shows direction. Scalar quantities such as mass, speed and energy have size only. Recognising force as a vector lets you predict whether an object accelerates, stays still or changes shape.
Students should be able to describe the interaction between pairs of objects which produce a force on each object. The forces to be represented as vectors.
Forces arise from interactions between pairs of objects, and each object in the pair experiences a force. In a contact interaction, such as a book resting on a table, the book pushes down on the table and the table pushes up on the book. In a non-contact interaction, such as two magnets or the Earth and a falling ball, the objects still exert equal and opposite forces on each other without touching. These paired forces act on different objects, so they do not cancel when you consider either object alone. Represent each force as a vector: an arrow whose length shows magnitude and whose head shows direction. When describing an interaction, name both objects, state the type of force, give its direction, and note that the two forces are equal in size and opposite in direction.
Your focus
- Define a force as a push or pull due to interaction with another object.
- Distinguish contact forces from non-contact forces and give examples of each.
- Classify a named force by identifying the interacting objects and whether they touch.
Show all 21 objectives
- Define a contact force as one that acts only when objects are physically touching.
- Classify given forces as contact or non-contact using the touching criterion.
- Represent a contact force correctly on a free-body diagram at the point of contact.
- Define a non-contact force as one that acts between objects that are physically separated.
- Classify given forces as non-contact using the separation criterion.
- Explain everyday examples of non-contact forces such as gravity, electrostatic attraction and magnetic attraction.
- Identify friction, air resistance, tension and normal contact force as contact forces in given situations.
- Describe the direction and effect of each named contact force.
- Explain why each named force cannot act without contact between objects or surfaces.
- Identify gravitational, electrostatic and magnetic forces as non-contact forces.
- Describe the conditions under which each non-contact force attracts or repels.
- Explain how non-contact forces act at a distance and change with separation.
- Define a vector quantity and identify force as an example.
- Represent forces as arrows with correct relative lengths and directions.
- Calculate the resultant of forces acting along the same line, including its direction.
- Describe the paired forces in a given contact or non-contact interaction.
- Draw vector arrows for each force in the pair with correct direction and relative magnitude.
- Explain why paired forces acting on different objects do not cancel each other.
Contact and non-contact forces exam tips
Marking Points
- Define a force as a push or pull acting on an object due to interaction with another object.
- State that forces are measured in newtons (N) and have both magnitude and direction.
- Classify contact forces as those requiring the objects to be touching, giving examples such as friction, air resistance, tension or normal contact force.
- Classify non-contact forces as those acting at a distance, giving examples such as gravitational, electrostatic or magnetic force.
- Identify the two interacting objects and decide whether they touch in order to classify a named force.
- States that a contact force requires the objects to be physically touching at the point where the force acts.
- Gives a correct example such as friction, air resistance, water resistance, tension, normal contact force or a push/pull from direct contact.
- Explains that if the objects are separated, the contact force is no longer exerted.
- Distinguishes contact forces from non-contact forces by referring to physical touching rather than distance.
- Applies the idea to a diagram or scenario, for example identifying the normal contact force between a book and a table.
- States that a non-contact force acts between objects that are physically separated.
- Gives a correct example such as gravity, electrostatic force or magnetic force.
- Explains that the force can still act across a gap without the objects touching.
- Distinguishes non-contact forces from contact forces by referring to physical separation rather than touching.
- Applies the idea to a scenario, for example a magnet attracting a paper clip from a distance or the Earth pulling on a satellite.
- States that a contact force requires the objects or surfaces to be touching.
- Describes friction as a force opposing relative motion or attempted sliding between surfaces in contact.
- Describes air resistance as a friction-type force opposing motion through air.
- Describes tension as a pulling force along a stretched string, rope or cable.
- Describes the normal contact force as acting perpendicular to a surface, supporting an object resting on it.
- Classifies each named example as a contact force and links it to a situation where surfaces touch.
- States that a non-contact force acts between objects that are not touching.
- Describes gravitational force as an attraction between masses, such as the Earth pulling an object downwards.
- Describes electrostatic force as acting between electric charges, with like charges repelling and opposite charges attracting.
- Describes magnetic force as acting between magnetic poles or between a magnet and a magnetic material.
- Explains that non-contact forces act at a distance through a field and weaken as separation increases.
- Classifies each named example as non-contact and gives a situation where the objects are separated.
- States that a vector quantity has both magnitude and direction, and applies this to force.
- Explains that the direction of a force is as important as its size when predicting its effect.
- Uses arrows to represent forces, with length showing magnitude and arrowhead showing direction.
- Combines forces along a line by adding those acting in the same direction and subtracting those acting in opposite directions.
- Distinguishes force from scalar quantities such as mass, speed and energy, which have magnitude only.
- Applies vector reasoning to a free-body diagram to decide the direction of the resultant force.
- Identifies that forces come in pairs arising from an interaction between two objects.
- Names both objects in the interaction and states the force each exerts on the other.
- Classifies the interaction as contact or non-contact, with a suitable example such as friction or gravity.
- States that the two forces in the pair are equal in magnitude and opposite in direction.
- Represents each force as a vector arrow with length for magnitude and arrowhead for direction.
- Explains that the paired forces act on different objects, so they do not cancel each other out.
Examiner Tips
- 💡Name the two objects involved in the interaction before classifying the force.
- 💡Use the test 'are the objects touching?' to decide between contact and non-contact.
- 💡Give a specific example, such as friction between a shoe and the ground, rather than only naming the category.
- 💡Underline the words physically touching in the question so you remember the defining condition.
- 💡When asked to classify a force, name the two objects and state where they touch before deciding.
- 💡Use a quick sketch with arrows at the contact point to support your written answer.
- 💡Ask yourself whether the effect would still happen if the objects were moved apart; if yes, the force is non-contact.
- 💡Name the two objects involved and state that they are physically separated in your answer.
- 💡Use examples from the specification, such as gravity, electrostatic and magnetic forces, to support your classification.
- 💡Name the two surfaces or objects in contact when justifying a contact force.
- 💡Use the word perpendicular when describing the normal contact force.
- 💡For each example, state the direction of the force relative to motion or to the surface.
- 💡State the two objects or fields involved and confirm that they are not touching.
- 💡Use repel and attract correctly for both electrostatic and magnetic examples.
- 💡Mention that the force decreases as the distance between the objects increases.
- 💡When asked to describe a force, give both its magnitude with unit and its direction, for example 12 N downwards.
- 💡On free-body diagrams, use a ruler so arrow lengths are proportional to the forces, and label each arrow with its value.
- 💡If two forces act along the same line, state the resultant and its direction rather than listing the individual forces only.
- 💡Use the sentence pattern: object A exerts a force on object B, and object B exerts an equal and opposite force on object A.
- 💡Label each vector arrow with the object it acts on, not just the type of force.
- 💡For non-contact forces, state the field involved, such as gravitational, magnetic or electrostatic, to show the interaction is real.
Common Mistakes
- Calling gravity a contact force: gravitational force acts at a distance, so it is non-contact.
- Thinking a force can exist from one object alone: every force arises from an interaction between two objects.
- Confusing mass with force: mass is measured in kilograms and is a scalar, while force is measured in newtons and is a vector.
- Saying that gravity is a contact force because objects feel heavy: correct this by noting that gravity acts at a distance and is non-contact.
- Confusing air resistance with a non-contact force: correct this by explaining that air resistance arises from collisions between moving surfaces and air particles, so it is contact.
- Drawing a contact force away from the contact point: correct this by placing the arrow at the surface where the objects touch.
- Calling magnetic attraction a contact force because a magnet must be near the object: correct this by noting that the objects are still separated and no surfaces touch.
- Thinking gravity only acts when objects touch the ground: correct this by explaining that gravity acts between any two masses at a distance.
- Assuming non-contact forces are always stronger than contact forces: correct this by stating that strength depends on the situation, not on the contact/non-contact classification.
- Calling air resistance a non-contact force because air is invisible; correct this by noting that air particles must collide with the moving object, so contact occurs.
- Drawing the normal contact force along the surface rather than perpendicular to it; correct this by drawing it at 90° to the surface.
- Treating tension as a pushing force; correct this by stating that tension pulls along the string towards the object at each end.
- Claiming that gravitational force needs air to act; correct this by noting that gravity acts through a vacuum, as between the Earth and the Moon.
- Saying that electrostatic force acts only between opposite charges; correct this by stating that like charges repel and opposite charges attract.
- Confusing magnetic and electrostatic forces; correct this by linking magnetic force to poles or magnetic materials and electrostatic force to electric charges.
- Treating force as a scalar and adding 20 N and 20 N to get 40 N even when the forces oppose each other; correct by assigning opposite directions and subtracting to find the resultant.
- Believing that a larger force always wins regardless of direction; correct by comparing the vector sum, since a smaller force can produce a resultant if the larger force acts in the opposite direction.
- Drawing force arrows with length but no arrowhead, or with an arrowhead but arbitrary length; correct by making length proportional to magnitude and always including a head to show direction.
- Saying the two forces cancel because they are equal and opposite; correct by noting they act on different objects, so each object feels only the force acting on it.
- Describing only one force, such as the table pushing up on the book, and omitting the book pushing down on the table; correct by naming both forces in the pair.
- Drawing both force arrows on the same object pointing in opposite directions and treating them as balanced; correct by placing each arrow on the object that experiences that force.